


Spontaneous Combustion

by skerb



Category: Undertale (Video Game)
Genre: Blow Jobs, Ecto-Genitalia (Undertale), Ecto-Penis (Undertale), Ecto-Tongue (Undertale), Fire, Gay Sex, Grillbones, Grillby's, Laughter During Sex, M/M, Monsters, One Night Stands, Oral Sex, Plot What Plot/Porn Without Plot, Sans Makes Puns, Semi-Public Sex, Sensitive bones, Soul Sex, Undertail, before i knew that was a thing, dom grillby, fire puns, health and fire safety violations, most valued customer, sansby - Freeform
Language: English
Status: Completed
Published: 2017-10-25
Updated: 2017-10-25
Packaged: 2019-01-22 21:14:53
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 8,774
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/12490992
Author URL: https://archiveofourown.org/users/skerb/pseuds/skerb
Summary: Summary:  Sans and Grillby’s flirtatious behaviour gets a little heated. Several health & fire code violations are committed as Sans finds out first hand how dominating a fire monster can be after staying past business hours.





	Spontaneous Combustion

**Author's Note:**

  * For [TychoAzrephet](https://archiveofourown.org/users/TychoAzrephet/gifts).



“so… what makes fire monsters tick?”

It had been an innocent enough question, one open to all kinds of interpretation. The bartender and patron getting along well - no one would suspect a thing. Not that anything was particular about the pair, but Snowdin was a small, sleepy town where not a lot happened, so gossip would spread fast. Hell, as soon as you stepped into the shop on your way through, the proprietress would immediately gab your ear off. And who else would likely be the subject of her interests, but the skeleton brothers?

One in particular, who had been jovial with the townsfolk around Grillby’s since their arrival, thought his private… more _intimate_ life, at least, should stay out of the limelight.

“Is this another one of your jokes?” was the reply from the brilliant bartender, who was half-leaned over the bar as he dried a pitcher while the last trail of monsters were leaving the bar, laughter amongst their parting conversations.

“not at all,” the skeleton replied, winking his good eye socket. While a permanent feature, his grin tugged mischievously, as though there was a punchline hidden in his smile. He leaned against the bar, swivelled in his seat as one of his slippers drooped lazily from his toes, catching it in time from falling.

Grillby gave him a pensive look. It wasn’t the first time Sans had been forward, but usually it was when he had far too much to drink. The fire monster tonight could, for once, count how many drinks Sans had partaken in on one crackling hand.

When Grillby didn’t answer immediately, Sans scooted his stool forward so it tilted precariously towards the bar, showing more interest at the other’s pause. “i won’t tell a soul, grillbz. monster to monster, everyone’s different. i’m sure you’re wonderin’ yourself what makes skeletons rattle-”

“A point of curiosity, I’m sure. What did you have in mind?” the crackle of his voice popped in interest as a cold pocket of air wafted in from the open door as the last (or rather, second last) patron left.

“i’ve got a bone of contention-” the skeleton supplied, poking his last bottle absent-mindedly with a slender digit, “everyone else is easy to figure out… and yet, i can’t figure you out.”

“Glad to keep your curiosity burning,” was the amused reply. Sans paused but broke out soon in a fit of snickers. Man, he _loved_ it when others humoured him with puns and plays on words. The thrum of his seat against the hard floor almost echoed against the continuous faint _snap_ and _hiss_  of his long-time friend’s warming demeanour. “So… what does makes a skeleton _rattle?”_

The jokester very nearly dropped the empty ketchup bottle in another fit - mostly a rush of heat that flooded his temples and the obvious double-entendre of a joke Grillby had put forth. Oh man. Oh man, oh god, oh _man…_ his thoughts echoed his slight anticipation and even embarrassment.

“Didn’t mean to put you out, friend,” came the level-toned crackle of Grillby’s voice from over him as Sans stooped for another bottle that had just rolled away from him. “Your words, not mine.” He enjoyed the flustered way the skeleton all but dropped his finger bones trying to collect his cool from an obvious reaction that even he himself hadn’t been ready for. Interest piqued, he almost seemed to purr, “Why don’t we have a little one on one?”

“aw, jeez grillbz… you honestly don’t gotta humour me-” in a last-ditch effort, the skeleton attempted to save face. If he was perfectly honest with himself - he wasn’t quite sure to expect from an evening with a fire monster. Over several resets, over what had to amount to several _years,_ Sans had to admit the notion of having a fire monster as a lover had… _intimidated_ him. While regular day to day interactions, casual, quick and fleeting touches, handshakes, bear-hugs… they just didn’t hold a candle to what could potentially go on behind bedroom doors.

Sans gave his skull a quick shake, lurching himself up and landing the bottle onto the bar top to an audible knock of glass against marble. He didn’t realise his mind and soul had wandered away in his thoughts, and he found himself grinning rather embarrassingly, realising Grillby looked expectantly at him. “sorry. what?”

“We’ve all but been dancing around these…” the bartender waved his hand, an ethereal-like movement that flickered light against the dim bar with such flare that it was hardly subtle, “...mutual escalating flirtations, I suppose we can call it? But, the thing is, Sans, and I’m sure you feel it yourself...” The skeleton was leaning forward again, hanging onto every deliberately pronounced syllable like a lifeline, “...that it’s time to jump out of the frying pan.”

“-and into the fire!” the words escaped Sans’ smirk without even thinking about it. While he half-wished he could take it back, he was kind of relieved at the other’s forwardness. It matched his own. “wow, um… yeah, with all the beatin’ around the bush here…” Absent-mindedly he raked his fingertips against his jawbone as though the action reflected his pensiveness. “can’t be my place; paps would throw a fit for uninvited guests so late.”

Grillby discarded the pitcher to his left on a shelf before his stance spread over the counter, returning a smouldering gaze level with Sans’. “Who says we must leave here? You’ve said many times you’re rather fond of my establishment.”

The skeleton felt the other’s heat reach his skull, flames near enough to gravitate between them; even though skeletons didn’t have a pinpoint feeling for heat or cold, Sans felt another rush thrum through him, snaking down to his soul where it pulsed in excitement.

“that i did. snow place i’d rather be… but here’s the thing, boss. i’m all for puttin’ on a good show for the audience, but that huge window is makin’ me feel a bit skeevy, y'know? it’s not exactly see-through, but monsters here would get curious enough to poke around, i’m sure.” He could not believe he just said that. Perhaps it was a defensive measure, but Sans oddly enough was feeling nervous. Not so much for his own well-being, but… hell, he had his pride in the fact his brother seemed completely oblivious of any of his other endeavours, in any timeline. That put a damper on things, for sure, if he found out...

With a flick of his hand and a rapt _snap_ of plasma fingertips, Grillby’s shutters hunkered down in a flurry of noise and clacks. There was a definitive _click_ of the door locking, easing Sans’ worries, if only a little bit. The thought of Sans being a private man when it came to lovers came to him nearly instantly; having known the skeleton for so long only made him more endearing.

“I have a suite,” was the only response he had, before leaning over the bar again and settling his lithe fingers under the skeleton’s jaw. He gave him a satisfied little grin, spectacles flashing with the wisps of flame that licked off his visage. He shrugged in a manner that indicated the door to the left of the bar. “Out the fire door.”

Sans all but melted into the touch, his hands attempting to find purchase against the smooth bar top, wanting more of that heat Grillby flooded his very being with. Another grin, and he murmured, the soft crackle of Grillby’s presence intoxicating him. “i’m not made of fire, man.”

“Your literal interpretations of my signage are endearing, Sans.” The fire monster’s digits rounded his skull, tracing over old worn surfaces and sending a heated shudder down the skeleton’s spine. Oh, but that _was_  nice. He tilted his skull, just a little bit, allowing a lick of flame to catch him under the collar. It swirled around the joint where his jaw met his vertebrae, and he sighed in appreciation. “i kinda like it when you say my name,” he murmured as his soul attempted to pull him forward for more touches, a subtle flicker beneath his hoodie echoing his anticipation.

Grillby was fully in front of him; both hands cupping his jaw to watch every nuanced expression the skeleton would reward him with. His bright eyes caught the accepting way Sans leaned into his touch and he couldn’t help but lean further down to join their faces in an approximate kiss. His own soul was flaring as it always was, but it was attempting to outdo itself as he flicked a tongue of flame over the skeleton’s almost slack-jawed grin.

A huff escaped the skeleton, the only evidence of breath was the short-lived wisp of steam that evaporated as it left Sans’ teeth. He’d reached for the bartender’s hands, then thought better of it and decided it was better grasp at his arms. He paused before settling at his vest, grasping the fine fabric between bony digits almost shakily when he felt another flood of heat trickle down his spine and his soul shuddered and pulsed hazy blue behind his ribs.

“that’s _hot.”_   His voice hitched as the fire monster took that as permission to continue, a pleased smirk on his vague features as he buried his face where most monster’s throats would be, blowing a current of fire against his clavicle. It glowed red, old debris igniting before turning to ash. Sans huffed out another breath, the dense heat both deliciously alive and unstable, creeping into every crack and pocket where it smouldered and popped. An unintelligible word or two fell from his mouth before his mind formed a cohesive train of thought, “i’m gonna fall at this rate-”

“You’re not going to Fall Down,” the fire monster replied against his jaw, precariously perched atop the bar as he slid one sleeve from Sans’ arm. His voice was laced with a harsher crackle than normally, perhaps irritation at Sans’ complaint? Impatience?

“no, i mean, _literally,_  i’m gonna fall off this damn stool-” His grip faltered as he teetered on his seat. Sans grabbed at Grillby’s vest with renewed determination as his soul was now pulling him more towards his partner and he had nearly lost balance. The fire monster chuckled, delicately removing the skeleton’s hands from his person and swiftly manoeuvred the bar top so he was now perched beside Sans on the next stool. He fed the other another kiss spiced with heat, no longer impeded by the counter, slowly and deliberately removing the skeleton’s hoodie and draping it over the bar.

Sans was still reeling from every touch, so much he barely had time to speak, let alone come up with any jokes relating to the situation. All he could was muster was a staccato of heavy pants as the fire monster quite literally lit his bones.

“Seems like you’re enjoying yourself?” It was a measured observance instead of a question and Sans almost violently nodded his skull, eye sockets blanked as another course of fire whispered against his other clavicle and the embers dropped down and trickled against his ribcage beneath his shirt. His laugh was rather short and distracted.

“that doesn’t u-um… oh _wow_ _,_  what’re you doin’...” was all Sans could muster before his hands felt their way up the molten core of Grillby’s body. His own involuntarily jolted when fire found its way under his rib cage, close to his spine. He jerked with a harsh gasp, not in pain, but surprise as he lowered his gaze to the bright lights and colours from Grillby’s reach beneath his shirt. Vaguely, somewhere in his addled mind, Sans realised Grillby had measured himself enough not to ruin his clothes, at least.

The fire monster had stopped, watching for any adverse reaction to his exploration, and when Sans had gestured that he was fine, he smirked in satisfaction. “A lesson. Fire monsters will fill their every pore with their magic… so that it builds in a dense, heavy pressure. So that there is no unoccupied space,” he supplied as Sans exhaled a hazy breath, the air simmering around them as though they were instead sitting by the lava floes in Hotland. He leaned in closer, so his eyes could watch the flutter of Sans’ soul through his shirt. “…Until our cores clash. Well, at any rate. You didn’t offer anything, so I had to make an educational guess. Was I off the mark?”

Sans doubled his expression, clutching at Grillby’s arms so suddenly the two of them lurched from their respective stools and into a crumpled heap on the floor. The smaller monster let loose a hoot of laughter, genuine and clear. It was a close call - any more to the left and Grillby would have landed on him. Just the opposite, the skeleton landed in a twenty-pound heap across the fire monster’s torso.

“jeez… nearly lost it there.” He gave Grillby a sheepish grin and pushed himself upright femurs at each side of Grillby’s waist, “b-but keep the fire a bit sparse around the ol’ ticker, eh? m'not built like i used to be.” To prove his point, he knocked twice on his sternum, the echo of the bones making him chuckle. “you gotta keep in mind a skeleton’s all bones - bear with me here.” Sans paused at Grillby’s derisive eyebrow raise, such as it was. “the more bones, the more magic supporting them, and… well…” Better to show than tell, and Sans hoisted his shirt over his skull, embers still attached to some of the more partial places Grillby had focused on higher up.

The fire monster drew himself up, placing a steady hand against the top of the other’s spine and shuffled so his back was against the bar. He watched Sans’ expression and the pliable smile on his face slackened at his touch, nearly allowing himself to sink against him. More appreciative noises sprung forth, almost held back. Well, that wouldn’t do, the fire monster thought. It was positively mesmerizing, as Grillby hadn’t been with anyone _but_ fire monsters - the choices were rather limited when one was a molten charge of onyx and perpetually burning.

Tentatively his hands reached for the column of stacked bones leading from his pelvis all the way up to his skull. The blue pulse under the skeleton’s ribs fluttered excitedly, a grand accompaniment to the dancing light of his body. It reminded the fire monster of the glowing crystals in Waterfall.

The most bones, hmm? The reaction was nearly instant, and for a startling moment Grillby thought he had caused harm from the sharp curse that left Sans’ mouth. The skeleton’s skull knocked against his brow in a semblance of reassurance while his voice huskily told him, “there you go. y'got the right idea. i’ll let you know if things get too hot to handle.”

He couldn’t help it, and he chortled as his digits attempted the few couple of stone buttons on the vest before being so rudely cut off. Sans’ body convulsed noticeably as the bartender traced around one of his vertebrae with both hands, each thumb and forefinger rubbing heat into his spine, between the spaces the magic keeping him from falling into a hilariously disassembled pile of bones. The heat teased at the sockets and his hands clung to the other as his mind effectively blanked, the wild heat driving him to shudder so hard it was difficult to stay upright.

Interested in the reaction, Grillby teased each one, deciding to start upwards, where there were definitely more bones, more things to explore and tease. Each new bone almost steamed with his heat and every touch sent a new wave of breathless gasps and pants from his partner. Barely a word was uttered, some started only to be singed away as the heat coupled with intense pleasure, as though the very notion of thought had been burned from Sans’ skull. The skeleton’s magic crackled in a mix of constraints as the flames had encroached on its territory, threatening to pull him apart. He felt like it was almost too much, his breathing erratic and hitched through arousal as his magic struggled to keep him together in several places.

Another one down, the fire monster had decided to trail one hand up to where he started while the other headed one down. A noise an octave higher than what Sans would have normally sounded like escaped him, his spine arching as he tugged mercilessly on the bartender’s vest, half-forming words and moans and what sounded like pleas for more. He was vaguely aware that his bones were smoking, were drying out, but there was no pain, no danger… he was in no state of mind to pay attention to anything but the delicious pampering of his partner, even if he rendered him completely shocked and begging for more. Because it was fire, _Grillby’s_ fire that was melding to his bones, and the thought chased through his addled skull the lower his partner’s hands roamed.

He was being filled with his fire.

His grasp popped a button free when heat met with the crest of his hipbones and he bucked his pelvis forward involuntarily, leaning forward to rest on his kneecaps. “ho-hold on, there,” his voice was excited but low, much lower than his usual register. “can’t… can’t help but f.. feel this’s a bit one-sided, y’know?”

Grillby let out a sound like the sound of popping kindling, almost reproachful as much as it was amused. “My curiosity got to the better of me.” There was absolutely no hint of apology. “You’re keeping yourself together, old friend.”

“barely… but m’all about.. education,” the skeleton, while suffering no lungs, made an effort to take a deep breath, “do i need to find a.. a blowtorch or somethin’?” He wasn’t entirely sure, hoping a joke would help soothe his anxiousness. The look on Grillby’s visage was a mix of interest and amusement, even though it was Sans’ way of using humour to diffuse his nerves and get a sense for what his lover wanted.

“That must be the single most lewd thing you have ever suggested, Sans.”

The skeleton’s bones rattled quietly as he moved up a little more, feeling the pressing heat around his hip bones as the other monster held him close. “i still really kinda like it when you say my name like that.” He felt as though he could get closer, though, needed it even. After all, there were so many spaces left for Grillby’s magic to reside. The thought sent a rush of want through him that made his soul pull him forward once again. Not quite pulling _himself_ _,_  but attempting to leave the safety of his person. His own blue magic coiled around his soul, anchoring it in place, only for it to come undone again by the fire monster’s whisper against his temple and two fingertips walk up every ridge of his spine. Every touch rewarded Grillby with a noted gasp.

_“Sans.”_

“so unf… unfair.. playin’ me like some kinda-” Sans’ mouth pressed against the other’s collar, muffling it as he bit into the fabric with a stuttered and frustrated groan. There was a joke there, but he’d lost it. Something to do with instruments… It was getting so difficult to focus, after all, and he could barely do anything more than attempt to get that damned vest off.

“Xylobone?”

A flicker of orange interrupted his vision as he realised Grillby had moved to give him another kiss when the other breathlessly laughed. The fire monster paid it little mind, interested in Sans’ blue light that was such a contrast to his own. The tendrils snuck around his rib cage as the skeleton pushed against the monster with greater fervour and took advantage of the paused hands to pop free the last of the vest buttons, tackling the bow tie and chemise next.

The air around them crackled and was alive as the skeleton pushed aside the taller monster’s collar, pleased with himself. His ulnae brushed around the other’s fiery frame and encircled his neck, yielding somewhat to what little weight he had and sending more of that heavenly sensation throughout his joints.

“pressure, eh?”

Grillby met the skeleton’s gaze with his own, a self-satisfied smirk on his face. “Like a thousand dying stars.”

“kinda puts a lotta  _pressure_ on me,” the skeleton noted thoughtfully, absently wondering if gravity and density were along the same channel. That would make his blue magic useful, and a mischievous grin tugged the corners of his face. “all right, bud. show me your core.”

“Filthiest thing you’ve said all evening.”

“i’m goin’ two-fer-two here, and all this lopsided skele-manhandlin’s gettin’ me hot and bothered.”

“Leave it to you to think of every bit of fire joke you can get your bony little fingers on, even now.” There was that amused, rich tone of his again, and Sans liked it.

“what can i say, i love playin’ with fire.”

If not for the seductive curvature of the other’s smile, Sans would have been more than a little surprised at the flared pulse from his partner. He allowed the other to cradle him with one arm as he inclined his posture to untie his black apron, then shrugged off the vest and chemise. It would have been a blinding sight for damn near anyone else; orange mixed with the palest yellow fire, wisps of blue-hot inferno rolling off Grillby’s torso. Sans watched the sight, perched upon the fire monster’s lap as his partner held a spread hand to his chest, the white flames rising like a wave from his centre. Tentatively he pulled something from the flames, now proffering it to the skeleton with a smouldering look.

“Careful now,” was all he added at Sans’ dumbfounded look, hesitating for a moment before he’d reached out to the shiny black orb. It was perhaps no bigger than the heel of his foot, and something electrifying shot through his bones when he made contact with it. It was heavy as he closed his fingers around it, so much the unexpected weight forced the much lighter skeleton forward and down. Before Sans took an ungraceful plummet into the floorboards, Grillby chuckled and cradled the bony digits surrounding his core for support.

“wow,” the skeleton turned his hands slightly, so the orb moved, the reflection and fractures of light glistening against the surface. “that’s.. somethin’ else. how’s it feel, big g?”

The fire monster shifted, the movement could have been taken as uneasy. “Like a monster I know is evaluating my prowess as an elemental.”

Sans flinched, suddenly anxious again. “didn’t mean it like that, grillbz,” he offered by way of apology. He swayed his skull from side to side briefly, as though mulling a thought. “y'know... i think i know what you mean. don’t think i haven’t caught you peepin’ my soul, boyo.” He leered, pleased with himself as Grillby looked slightly taken aback. “can’t say i blame ya. a skeleton’s light show is probably the best ever seen, as far as non-elementals go.”

Grillby crackled in contentedness. “It was interesting. So much different from my own.” He paused as he pulled the skeleton closer, using one arm while the other laced his partner’s digits with glowing embers. He saw the shudder, the quickened beat of Sans’ soul quivering with his movements and words. “I liked how our magic interacted.” He wanted more of it, even.

Sans nodded in agreement, lightly caressing the orb suspended between them.

“I liked how your magic tried to resist mine,” the fire monster continued, one finger sensually tracing a line down Sans’ femur from his hipbone. His eyes flicked up to the skeleton’s sockets, measuring his reaction and relishing the way the minute presence of his fingertips lightly scratched against his core in response. His heat flared with it and his voice rolled into a purr. “Yet I could also tell it wanted to accept me.”

“i’m.. i’m 100% not jokin’ when i told you skeleton bones are held to.. together with magic,” Sans fumbled over words when his partner’s hand dipped between the void between his shorts and pelvis. “th… _hnn,_ that your magic was teasing apart… and my magic w.. was… was…” Distracted by the heated magic that threatened to undo him again, the skeleton bucked his pelvis forward again, his spine hunched in barely-contained restraint. Any more of this and he’d likely break like a wound up toy well past its prime. He needed something to level the playing field.

At Grillby’s next pause Sans chanced his stamina and surrounded the core in Blue, giving a start to his partner, and in return making the skeleton anxious. “wait, was this not right?”

“It was only sudden,” the fire monster was quick to explain, although the gravitational pull he felt around his core made his flames spark and whip excitedly like a cat’s tail, flicking colours of different intensities. Well, he mused thoughtfully, at least Sans had been right about the window…

“i can’t hurt you like this, right?”

“Asteroids have failed,” was the amused reply as Grillby leaned against the bar, watching the licks of blue fire wash over Sans’ hands, then over to his soul, which closely matched the core. Interesting, considering the bluest flame was hottest, and so resembled Sans’ natural charisma.

“pressure, right?” the skeleton drew in something akin to a deep breath, steeling his nerves and watching for Grillby’s nod of approval. Once given, the skeleton gave the other a sly grin, turning the orb over a little more easily in his hands, thanks to the Blue. “well then, i hope you won’t be too hard-pressed to tell me if things start t'go sideways-” Okay, that one was a stretch, but it elicited a chuckle anyway.

Blue was easy to manipulate, but usually Sans just wasn’t in the mood to experiment further than grab A, fling A to side, or grab B, hold B in place while X… Never had he _held_ something with Blue, clasped his hands around that something, and forced his magic to close over it, and… squeeze? Really? The thought made the skeleton feel somewhat lascivious... that what he held in his hands was something akin to, what? Magical monster parts? He felt his magic twist languidly near the base of his pelvis where Grillby was idly stroking while he watched, both distracting him and sending a tightening surge towards the fire monster’s core.

The reaction was nothing short of electrifying. While the orb didn’t yield, Sans caught the tumultuous combustion, the scratch against the flare of his hip bones in response. The corners of his grin heightened slightly as he closed the small distance between them, pouring his magic against the smooth surface in his hands, wanting another reaction like that. The air crackled and sharp pinpoints of light sparked around them as the air literally ignited with the bartender’s appreciative noises. Each fiery throb echoed through his frame as plasma coiled from the core and around his fingers, bathing him in a sensual glow.

“everythin’ ok, big guy?”

The taller monster exhaled a raspy hum, each of his hands trailing from Sans’ tibiae to his kneecaps, cradling the bends where his flames would penetrate. _“_ _Perfection, Sans,_ _”_   was the heated response, and the skeleton shifted as his magic wavered, his lover’s tone flooding him like nothing else. He faltered, was concentrating on the tightening pressure against the fire monster’s core when he realised, much to his distant amusement, the magic in his left leg had become overwhelmed very quickly and that the socket was now free.

Off balance, Sans threw his left hand out to steady himself, grasping for Grillby’s smouldering shoulder with a couple of measured breaths. After a moment and with Grillby looking at him in that way, he chuckled as the other brought the slipper-sporting appendage into view.

“i’m literally fallin’ apart here, grillbz! Wh-whoa, whaddaya doin-” The fuzzy slipper fell from his toe as Grillby brought the tibial head to his glowing mouth, white-hot tongue flicking over the end as Sans quivered in his lap, struggling to maintain composure, concentration, and most importantly, balance.

He rattled as he straddled the other’s hips, squeezing his left side with his bisected femur and watching the captivating sight. That tongue left a trail of iron-hot embers in its wake, how it dipped down between the two parallel bones. The smaller monster tightened his hold against the orb with his magic, however it was pooling more heavily inside of him, and it looked as though Grillby was having fun toying with him and watching him squirm in pleasure. The glow was denser, throbbed harder while watching the other leave teasing bites and scorch marks on his bones; it was almost _overwhelming._

Yearning for something more, Sans went for the other’s pants waist with one hand, pulling the band and buckle of his belt in earnest as the other watched with interest. He was rattling louder now, the sound echoing in the empty bar, a fine accompaniment to the hiss and crackles of his partner. His glow pulsed brilliantly as Grillby moved to encircle his frame, hand hooked onto one of his ribs at the opposite side as he lifted him gently. The other hand discreetly lowered his lower leg to the floor and snagged his short’s waistband, tugging it down. It was easy enough to discard with half a leg gone; Grillby simply twisted it between them and down Sans’ right leg, an eruption of chuckles escaping him as the skeleton moved to help him with his own trousers.

With everything off of the smaller monster, Grillby’s gaze settled at the coils of dense magic at the base of Sans’ pelvis, curiosity and admiration for the lights… but also for the shape that was attempting to take form. Without pause, he reached his hand towards the mesmerising mass, blue tendrils leaning towards his touch in a trembling wave as the skeleton followed their direction and nipped coyly at the base of his neck. Flames dragged through his teeth and tumbled down his rib cage as Grillby’s hand teased a desperate moan from him.

His eye sockets blanked as his magic moved to be guided by his lover’s hand, leaning forward, grasping at his form and attempting to find something similar for Grillby to experience. He huffed almost impatiently, flames wisping from his mouth along with a raw, frustrated swear. He put his weight into his right side, tilting his body so he could better squeeze his digits into the front of the fire monster’s pants. He was rewarded with a hiss and another bite to his clavicle, where it sizzled and started to turn dark, eliciting him to probe further and give Grillby as much pleasure as he was receiving.

“it… it feels like you’re cheatin’,” the skeleton gasped, holding his fading Blue magic against the fire monster’s chest and giving the orb a feeble, physical squeeze. It was beginning to get difficult, holding himself together, his arousal, _and_ keeping his magic on the core. He’d already lost a leg; would he lose an arm? Somewhere at the back of his mind, there was a chuckle at the joke of paying an arm and a leg for a smokin’ hot date.

There was something measured, yet wild and unyielding in Grillby’s tone as he replied, “I pride myself on being an excellent host.” He pampered the tactile density responding to his body, relishing every subtle shift in the other’s magic as Sans involuntarily moved forwards at his administrations.

 _“ahn…_ this’s… this must be what you call 'happy hour’, eh?”

“You never do run out of material, do you Sans?” The tone was teasing as Sans’ bones clacked and pressed against him.

“ahh.. hahh… at this rate… i.. i _might.”_   It wasn’t that far off - not that Sans ever would run out of jokes, but it was becoming increasingly difficult to focus on anything else than what was happening right then. His magic had condensed so thickly that Grillby had a firm grasp of it, physicality quite literally played into his hands at long last.

He groaned into Grillby’s shoulder as the fire monster thickened his hold on his newly conjured appendage, so unstable and new - well, at least in _this_ run. And what a dry run it had been, something in the back of his mind remarked. He hitched himself up slightly to meet his brow to Grillby’s again, looking down between them at his heavy erection, thick in magical heat.

“Mind giving me a hand?”

Sans’ body shuddered at the heat encircling his cock, huffing out another laugh as he breathlessly replied; “grillbz, man, you already have a leg-up on me.” He relented as his hand probed further into the front of the fire monster’s pants, easily finding his partner’s own heat and pulling it free from the zipper’s opening. A delicious wave of magic crawled up his spine as Grillby’s fingertips traced the length of his shaft and around the tip and he shuddered in delight.

“What other surprises do you have in your reserves?”

The skeleton had an idea or two, his magic circling his bones and mingling with the remnants of fire the other had left from his earlier petting. The wisps travelled up his vertebrae and into his skull where his left eye light lit in bright cyan and yellow. His grin grew a little more mischievous as his normally fused teeth parted, revealing a wide, inviting tongue.

Well, this proved to be more than a little more interesting, the fire monster thought. His other hand still firmly grasped at one of Sans’ ribs, he pulled the skeleton closer to press their mouths together in another, more passionate kiss. The tongue was useful, even wild as Sans tried to outdo him - however, fire monsters were known for their dominance, and Grillby used every trick in the book to come out on top. His own tongue coiled against the other’s as his hand teased another moan from the skeleton. In return, Sans pressed his core against him where he kept himself stable, his other grasping and pumping his cock in an effort to dominate.

The air around them danced as Sans broke off the kiss but wasted no time in dipping his skull down, his broad tongue swirling a languid, sizzling trail of saliva against the burning mass before its remnants evaporated. The heard the crackled huff of his partner and the loosened grip at his pelvis momentarily before he stooped lower, the movement tricky with no support on one side. He left the core suspended in Blue, both his hands free to scoot himself backward enough to lean down further to reach the other’s cock with his tongue.

The reaction was priceless. He wasn’t quite sure how it’d work with a fire elemental, but the conjured tongue definitely did the trick. He had no way of knowing first hand the domineering trait of fire monsters, but suffice to say it would have added to his ego the way Grillby arched against the bar, releasing his rib but only to hook his fingertips into his shoulder blades. The sounds were a heated mess of snaps, shaky shudders and fiery groans as his tongue played around his partner’s heat. It made his own throb lustfully, forgotten for the moment as Sans took his partner into his mouth.

As the sudden light flared, Sans thought his actions had blinded him. His skull filled with an inferno and clashed with his magic, his conjured tongue coiling and stroking as Grillby helplessly bucked beneath him, fingers scoring burn marks into his poor bones. Steam and pale flames were erupting from his nasal cavity and eye sockets and the fizz of ectoplasm filled the air.

Not wanting to be beaten, the fire monster pulled the other up, his frame wracked with lust and wildly flickering energy. Sans thought he was going to have some kind of meltdown, his flames looked so erratic. He allowed the other to pull him forward, his hands hooked his fingers into the voids of his ischium, teasing the remaining magic that pooled there, realising just where his pelvis was hovering over…

Sans’ tongue was caught into another deep kiss, leaving no room for him to think of any joke or tease to tip the scales into his favour again. Grillby pulled on the voids, closer to him, the tip of his erection teasing the mass of magic at Sans’ tailbone. _Oh wow, this’s actually happenin’,_ Sans thought, allowing the fire monster to lead his body while craning his skull to watch from above. He huffed as his magic instinctively sorted itself, primed itself for the invasion of heat and flame.

The first insertion had Sans rake his fingertips against Grillby’s torso, his magic gripping and roiling as flames licked him. He let out a surprised groan as the full expanse of his partner’s girth pressed through his magic and against his tailbone, lighting him a long way up. He screwed his eye sockets shut, both wanting the pressure against his tailbone to cease, but wanting it again. Urgently, he attempted to move up, but Grillby kept him arrested.

Quizzically, the skeleton looked back up to his lover’s face, attempting again, but caught that teasing grin on the other’s visage. His magic was still swirling around his bones, aching with the severe need to continue.

“grillbz…. c.. c'mon now,” the skeleton almost whimpered. When the other’s grin grew wider when Sans attempted another tug upward. “oh c'mon…” He felt almost nervous? “d… don’t make me beg.”

The fire monster’s chuckle teased against his sternum in a lusty prowl, a wake of fire kisses popping against his bones. “Then I won’t.” With that, he shifted the skeleton up, watching his eye lights as he lowered and thrust again.

Sans’ eye lights flickered hazily with the contact to his tailbone and he clutched at the fire monster’s chest again, unable to find purchase. A startled sharp gasp and a curse escaped him, his conjured tongue dissolving momentarily as he bit down on it.

“Language, Sans,” the bartender purred, singeing the bones under his skull.

“ta.. tailbone,” was all the smaller monster could muster, shaking with effort to keep himself upright.

A devious smirk passed Grillby’s vague features, pulling the skeleton up and closer to him and taking one of his arms and hanging it over his shoulder where Sans could properly hold. “Your tailbone, you say?” He angled the skeleton’s pelvis before he pushed through the dense magic again, aiming right for that spot. It was easily seen, after all, and Sans’ response was loud, almost strangled, and his groans of appreciation were very, _very_ enthralling.

The smaller monster’s magic around the core was faltering with each thrust, but came back strong in recovery every time he was lifted upwards. The tips of his bony fingers dug into the plasma of his lover’s chest surrounding it, huffing as the heat swirled torrentially with every thrust. The fire monster’s momentum regulated, pulling a string of desperate pants and groans from the both of them. Due to his nature, the bartender enveloped the smaller monster with one arm, holding him closer while his hand kept firmly pumping his erection.

He watched his lover’s eye lights, how they’d flicker and fade. They had dragged their way to his direction from watching their amorous activities. From the delightful colour of his magic to the gentle, yet raging hue of it, Grillby was enamoured. A passing thought flitted through his mind, sensuality building its own charisma as the other’s magic had begun to quiver and clench between their hips. A skeleton’s light show _was_ something impressive… and a fire monster’s love left marks of its own.

He rather enjoyed it - the subtle way Sans’ bones fissured and relented to him, his heat’s pressure excitedly exploring places where less intrusive monsters could not go. He felt as though it was something only he could do, to wrap every infinitesimal space with his core’s fire, welding with the other as much as it could. The glow of the other’s bones was _gorgeous,_ so much he couldn’t help but send the compliment through a breathy, moaning sigh against the other’s mouth.

He felt Sans shudder against him, those bony fingertips grasp his insubstantial frame. He responded to the compliment in such a beautiful way, his soul’s glow swirling, fluttering like wind through high grasses. The representation of his soul was out of hiding, although not too far from his sternum, looking so perfect, yet so small. It really was beautiful.

And yet still the skeleton had to make a joke of it, even between hazy, moaning grunts and gasps as he bucked his hips along with the fire monster’s rhythms. “heh… _hah_ , and you’re the light of m’life.”

Grillby beamed in response, gentle mirth echoing through his tone as he hitched the other higher and against him so his fingertips could play against the ridges of his spine again, to kiss the skeleton with as much passion as he felt in that moment. He felt his soul echo the other’s sentiment as Sans’ breathy voice complimented their exertions, each teasing rub and flicker of flame against his vertebrae and cock a punctuation of how close to the end of their evening they were approaching.

The fire monster admired the other’s stamina throughout - while nothing to balk at, given enough fan and fuel, a fire would last _months_ burning, and he could tell the skeleton was giving it his all with the limited information he’d given him. The thrum on his core was like a heartbeat, and while gentle, filled him with affection. He cherished the feeling, his bright eyes snapping closed as he felt Sans’ magic enclose around him like a vice. It was unexpected, although he had suspected something as Sans’ magic had become more and more dense the more active his thrusts became.

He suspected Sans was approaching his breaking point, a noise accompanying his staccato, haggard breaths as Grillby redoubled his efforts. The begging, the pulling at his shoulders - the fire monster relished them. As with the nature of his kind, he had successfully dominated the other. The clatter of bones accompanied the odd noises Sans made as he shuddered, fixated on that portion of word that just refused to pass his teeth.

Grillby nuzzled the skeleton’s temple, his own movements becoming desperate. _“Ta?”_ he repeated inquisitively, breathing a plume of wispy heat at Sans’ shoulder plate.

The skeleton nodded feverishly, his bones rattling as though they were threatening to snap apart. “ta.. hah.. _ta… tail..bone…_ _”_   he barely gasped out, as the taller monster’s thrusting had been ceaselessly knocking into one of the most sensitive places in his entire being. Any more and he doubted he _would_ be able to keep himself together, let alone last any longer. It was wishful thinking that he could, as the fire monster had taken what he had said into careful consideration, providing long, hot strides along his cock and the innermost side of his tailbone.

His magic roiled erratically within his pelvis, both in a heavy storm of ceaseless pleasure and release as he felt he could take all that he was able to. His orgasm came crashing over him with a strangled, muffled moan into the inferno of Grillby’s shoulder, magic riled up so hot and so quickly that it seized around the other monster’s heat, pulling a haggard groan from his mouth and a flare as he released against the inside of Sans’ spine.

The plasma coated and slid down the ridges, seeping into the cracks where it glowed like molten iron and Sans slumped against the taller monster’s frame with another shudder, the need for fictitious breaths overpowering him.

After a long silence of just being held and of the other’s comfortable heat, Sans dared to raise his skull from the other’s shoulder, still rattling in exertion. “h… holy.. _smokes...”_

Grillby smirked at the compliment, one of his bright eyes fashioned a wink. “I trust it was to your satisfaction?” he teased, and Sans grazed his digits against his torso lazily, but couldn’t hold back a chuckle, smoke leaking in subtle wisps from his teeth.

“more than excellent,” the skeleton huffed, “and i didn’t have to pay an arm and a leg.”

“Just a leg.”

“easily fixed once i, uh…” he trailed off, his grin tugging tighter at the sight of the amputated leg to his left and he couldn’t help but laugh, the wholesome mirth shaking his whole body. “i got carried away.”

“Things got heated quickly,” the fire monster supplied, that teasing, sensual tone still present as he shifted the smaller monster against him.

“who would’ve known i turned out to be such a hot head?” the skeleton mused. He wrapped his arms around the other monster’s shoulders, sinking contentedly into his warmth. And for a while they stayed as they were.

It was not until Grillby noticed the skeleton’s longer silence that he nudged him carefully with his face, depositing a kiss to his skull affectionately. There was a minute grunt that signalled that yes, Sans had taken a moment to rest his eyes, and he slowly leaned back off Grillby’s chest, definitely waking up. The fire monster found that endearing, in a way.

“It’s late,” the fire monster murmured, a hint of regret in his tone.

“five more minutes,” the other replied, his voice quiet and betraying his fatigue. Grillby hummed quietly, holding the smaller monster with one arm while he reached for his leg that remained on the floor beside them, toes flexing languidly. Gently, he introduced the limb to its socket, nudging it patiently until it clicked into place. His fire still burned in the creases, but Sans’ magic held it in place.

“glad you’re here to pick up the pieces,” the skeleton’s voice was muffled against his shoulder. He remained there a little longer, comfortable and content. The fire monster watched the simple movements his partner made while fending off another nap, before quietly nudging him again.

“I’d hate to be the bearer of bad news-”

“mhm.”

“-And I feel tactless for mentioning it, however,” the fire monster’s tone was delicately precise and warm, and Sans felt he could get used to falling asleep to it, “Generally, if you are out too late, your dear brother tends to come calling…”

Well, that was enough to get the skeleton’s attention. He lifted his skull and cracked his look to the door almost in a panic. Then he relaxed, his bones trembling in relief. Oh, good, he thought. No Papyrus. It wouldn’t do to keep him waiting, after all… He groaned in remorse at the other’s good intentions. He settled down a little, then blinked as he saw embers drop from his skull.. and hands… and… oh jeez. Inquisitively, he looked at his fingers, fire lit between the joints like glowing coals.

“m'on fire, man,” he said dumbly, unable to find a joke for once.

Grillby’s body pulsed, almost as if in pride and affection. “Filled with my fire,” he almost purred.

“that’s kinda hot.” There’s the joke, Sans snickered to himself.

“Your house is made of wood, is it not? Best to cool off,” the fire monster supplied, pausing for a moment with a soft sigh of smoke, as though regretting his suggestion. “You look so stunning this way, too.”

“m'sure paps would have a load of q’s if i snuck in late at night _and_ burned our house down…” Sans’ grin relented slightly, his expression softening as he scratched absent-mindedly at his skull. “as much as i hate t'dine and ditch, i guess i should go before he breaks through the window lookin’ for me.” He paused. _“stunning?”_   His grin returned, although finding himself somehow.. bashful.

“Radiantly incandescent _._ ” Grillby corrected, his stature positively affirming his affection.

“there you go,” the skeleton chortled and moved to get up, bones popping and his legs quivering through their prolonged strain.

 

After the two had redressed each other, Grillby met the skeleton at the back of the bar, turning the knob of the fire exit. At Sans’ quizzical look, he gestured, the movement elegant and almost grandiose.

“You _are_ on fire, after all.”

Zipping up his hoodie, the skeleton made his way over, bony feet kicking his slippers back on with practised ease. His walk had a little jaunt to it, and couldn’t help but send a wink Grillby’s way as he approached the door. Before he left, however, he hesitated. He wasn’t good with goodbyes, only see-ya-later’s. So it was to his relief that Grillby took action for him, leaning down to cup his skull in both of his hands and give him a goodnight kiss.

He dwelt on it as he walked home, meandering around the side of the bar. There was no one around - everyone was in bed in their cosy homes, completely unaware of the utter health code violation that took place at Grillby’s. The sign flickered as the light turned off for the night, drawing his glance upward, where he caught his reflection in the windows. He stepped forward, eye lights turning to pin pricks as his hands flew to his skull, eye sockets showing a smouldering, old red ember from within like a dying forge.

“hah… heh. man. i better cool my head…” he murmured under his breath, although was interested in the way steam emanated off his head in the dense chill. Warily, he looked around, then proceeded to wander to a rather large snowdrift about twenty feet away from the establishment and fall back into it. The snow melted on contact with a loud sizzleand he sighed almost lamentably as the glowing heat dispersed from his skull and bones.

He lay there for probably was more than warranted, closing his eye sockets. Might as well catch a few z’s while he was out.

 

“SANS!”

He would have darted up from the short nap, but instead lazily opened one eye, blearily looking up at the tall skeleton that was all too familiar to him from the snow. He didn’t know how long he’d been out, but the inch-high layer of snow covering him told him it had likely been a while.

“oh, hey bro.”

“DON’T 'HEY BRO’ ME, BROTHER! WHY ARE YOU NAPPING ON THE SIDE OF THE ROAD?”

“y'could say i was bone-tired?” the smaller skeleton answered lamely, not bothering to stifle a yawn as he allowed his brother to tug him from the snow pile with ease. He huffed when Papyrus exasperatingly lurched him over his shoulder, knowing full well he’d get an earful at home. Oh well, that meant he could sleep a little more until then, at least.

 

He wasn’t out long before he was roused by the jingle of their wreath on their door, bright lights flooded his eye sockets and he blinked when Papyrus set him down. Welp, he braced himself for it, shoving his hands into his hoodie’s pockets and closing his eyes again. He was the picture of innocence; he had done absolutely nothing wrong.

“SANS, I-”

Well, that was short. Sans had been ready for far worse. He lazily opened his right eye again, the blur of light behind Papyrus obscuring what was already an extreme overreaction. He jerked forward as Papyrus seized him by the collar of his hoodie, his eye sockets wide and an aura of brotherly protection gearing up into maximum overdrive.

“sup, bro?”

“SANS… WHAT HAPPENED? YOU’RE COVERED IN SOOT AND… MARKS!!!”

Oh.

_Oh._

Sans’ grin tightened artificially as he shrugged it off, his mind searching for a joke.

“would you believe me if i said i’ve been moonlightin’ as a chimney sweep?”

“SANS…”

“`sides, no one else is doin’ it in town. oh, what is a chimney sweep’s most common ailment?” At this, his brother’s eye sockets narrowed in peevishness and he let go of his hoodie, his arms flying to his sides and his hands in fists.

“BROTHER, IT IS ENTIRELY TOO LATE FOR THIS!”

Sans could barely contain himself. “the flue!”

His laughter echoed down the living room as Papyrus stomped off towards his bedroom while throwing his hands in the air in barely suppressed irritation. Moments later, his door slammed and Sans chuckled quietly to himself. Heh, the flue. He perked slightly when he heard the door reopen and his brother belt out;

“SKELETONS CANNOT GET THE FLU!” Then another, more quiet slam.

**Author's Note:**

> Inspired by "In From the Heat" by TychoAzrephet since I love Sansby and wanted them to have a hot night out and Ty is a bad influence. ♥ Also a few pieces of fan art drawn by some great people on tumblr made my brain happy and a pile of mush! Such inspiration :'D I hoped you liked Grillby using Sans as his personal forge, snicker.


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